1. Field of the Invention
This invention relates in general to document processing systems; and more particularly to overlapped document detectors in such systems.
2. Description of the Prior Art
Document processing machines often serially process documents from a stack of documents. Generally, a mechanical feeder mechanism is used to serially feed documents into a document transport path at predetermined intervals. For various reasons, such feeder mechanisms can accidentally feed two documents such that they physically overlap to some extent rather than being serial. It is, of course, desirable to detect such overlapped documents when they occur because generally the machine will not be able to process them properly. Once the overlapped documents are detected, the machine can be automatically stopped, or the machine can continue to process documents with the overlapped documents being automatically processed into a reject pocket. Generally, document overlap detectors are located immediately after the mechanical feeder such that overlapped documents can be detected before other document processing functions are performed.
Document processing machines can process various types of documents such as, for example, bank checks. Thus, documents can have various sizes, various thicknesses and other irregularities such as lines, printing or pictures thereon or punched holes therein.
One technique that has been used for detecting overlapped documents utilizes vacuum principles. Vacuum pressure from ports located on either side of the transport path pull overlapped documents apart and then the individual documents can be detected. One such system senses changes in vacuum pressure when the separated documents cover the vacuum ports on either side of the transport path. Such a system, however, requires an expensive and noisy vacuum pump together with cumbersome fluid plumbing. Also, such a system will not work on documents that are stapled or otherwise firmlyfastened together.
Another technique for detecting overlapped documents utilizes positive air pressure principles. Positive air pressure is blown on the documents in an attempt to separate the documents and press them against opposite sides of the transport path. Light sources and sensors (in pairs on the same side of the transport path) on both sides of the transport path detect light reflected from the documents when they are against the sides of the transport path. This technique also involves a pump and fluid plumbing. Also, it will not work on documents that are stapled or otherwise firmly fastened together.
U.S. Pat. No. 3,778,051, issued to Allen et al, describes a mechanical transducer that physically contacts documents for detecting and indicating when the thickness of successive documents differs from the thickness of a first document. Such a system, however, must physically contact the document and operates only on a sequence of documents having the same expected thickness.
U.S. Pat. No. 3,186,708, issued to Hinz, describes a document overlap detecting device that utilizes light reflected from the leading (or trailing) thickness edge of an overlapped document. The device provides two of these thickness edge detectors and requires coincidence to indicate an overlapped document. Such a scheme, however, does not detect the translucency of documents. Also, the edges of typical documents might not be predictable enough to reliably reflect the light as desired. Also, such a detector could be fooled by creases or other irregularities in a document's surface.
U.S. Pat. No. 3,026,419, issued to Aweida et al, describes a document overlap detecting device that utilizes a single sensor to detect the translucency of a document. The single sensor is used to detect sudden changes in document translucency and thus indicate overlapped documents. However, such a device could be falsely triggered by marks on or holes in documents.
U.S. Pat. No. 3,578,315, issued to Milford, describes a document overlapped detecting device that utilizes two document present sensors and a third sensor that detects diffused (refracted) light that passes through translucent portions of the document. The device requires coincidence between the two document present sensors and the diffused light sensor. At the beginning of each batch of documents, an automatic gain control amplifier on the output of each sensor is adjusted for that batch of documents. Thus, this device appears to operate on documents that are similar within a batch. Also, this device could be falsely triggered by marks on documents.